1. Field of the Invention
The present invention generally relates to optical filters, and more particularly to improved accuracy filtering devices including Bragg filters incorporated in optical waveguides, especially in optical fibers.
2. Description of the Related Art
The advent of wavelength-division-multiplexing (WDM) as the technology of choice for upgrading the capacity of fiberoptic networks has created a demand for devices and components capable of pushing that capacity to its limit. Fiber gratings are well suited for the challenge, due to their inherently narrowbandxe2x80x94almost squarexe2x80x94wavelength response function. Fiber gratings also find other uses in communication systems, for example, as gain equalizers, where their spectral profile is made to match the gain curve of an erbium-doped fiber amplifier. Linearly chirped gratings are also used as dispersion compensators. In these applications of FBGs, a problem arising from changes in the surrounding temperature has been observed. Because the spacing of Bragg grating determines the central wavelength of the reflected optical signal transmitted in an optical fiber, the FBGs are carefully designed and accurately manufactured. The problem is that the optical fibers elongate in a raised surrounding temperature so that the reflected wavelength deviates from the design value. These variations which can be as small as 50 GHz (0.4 nm) are undesirable for the narrow channel spacings used in high-performance systems. Thus, reducing the thermal variability of FBGs is a key to commercial success in the telecommunications industry.
FBGs can be fabricated by interferometric or phase-mask techniques. However, packaging is a vital technology, which makes FBGs suitable in real-world applications. Baking, laser welding, epoxing, and re-coating can result in the deviation of desired central wavelength. Thus, a packaging device designed with a post-tuning mechanism is necessary. To compensate the FBGs thermal wavelength shift, a mechanism that has positive and negative thermal effects is desirable. One of the methods to achieve this object is to include a press-stressed mechanism in the packaging device.
There are already known various constructions of optical filters, among them such which utilize the Bragg effect for wavelength selective filtering. U.S. Pat. No. 4,725,110, issued on Feb. 16, 1988, discloses an example of a method for incorporating an optical filter of this type in an optical fiber. This method involves imprinting at least one periodic grating in the core of the optical fiber by exposing the core through the cladding to the interference pattern of two ultraviolet beams that are directed against the optical fiber at two angles relative to the fiber axis that complement each other to 180 degree. This results in a situation where the grating is oriented normal to the fiber axis so that it reflects, of the light launched into the fiber core for guided propagation therein in a propagation direction, only that having a wavelength within a very narrow range, back along the fiber axis opposite to the original propagation direction so that such reflected light is guided in the core to the point at which the original light had been launched into the fiber core. On the other hand, this grating is substantially transparent to light at wavelengths outside the aforementioned narrow band so that it does not affect the further propagation of such other light. The incorporated periodic grating of this kind thus produces a narrow transmission notch and a commensurately narrow reflection peak in the spectrum of the light propagating in the optical fiber in one or the other of its longitudinal directions. The frequency of the light affected in this manner by the incorporated periodic grating is related to the periodicity of the grating in a manner explained in the above patent.
The optical fiber with the incorporated grating filter obtained in the above manner is well suited for use as a strain or temperature sensor because the frequency of the light reflected by the grating varies either with the strain to which the grating region is subjected, or with the temperature of the grating region, in a clearly defined relationship, which is substantially linear at least within the range of interest, to either one of these parameters. It is also possible to employ this kind of a sensor in an environment where both the strain of the grating region due to external forces imposed on the fiber, and the temperature of the grating region, vary with time in a manner that is not necessarily concurrent, and to separately evaluate the reflected wavelength changes attributable to the grating region strain, on the one hand, and the grating region temperature, on the other hand, in a manner that is also discussed in the above patent.
As advantageous as the incorporated optical core grating filter of the above type is for use in the above and similar applications, there are other applications which would greatly benefit from the use of such a filter but for which the above filter is not suited in its basic form disclosed in the above patent, for the very reason that enables it to serve as a temperature sensor, that is, the temperature dependency of the wavelength of the light reflected thereby. Inasmuch as the frequency of the light reflected by such optical filter varies with the temperature of the grating region, this basic filter cannot be used in applications where the reflected light frequency is to be independent of temperature. This precludes the use of the basic filter as a frequency standard and in similar applications.
U.S. Pat. No. 5,042,898, issued to Morey et al. on Aug. 27, 1991, discloses a cylindrical package comprising two materials with different thermal-expansion coefficients. The changes of fiber longitudinal strain can be compensated by a grating embedded component. The disclosed temperature compensated optical waveguide device is based on the concept that changes or shifts in wavelength attributable to changing optical grating strains can be used to counteract and/or eliminate shifts in wavelength resulting from variations in the optical grating temperature. For example, a constant wavelength of reflected light may be maintained during a drop in temperature by increasing the longitudinal strain on the fiber, and vice versa. In the compensation device described in U.S. Pat. No. 5,042,898, a portion of the optical fiber containing the embedded grating is sectioned off by securing the optical fiber at each side of the grating to separate metallic compensating sections arranged for longitudinal movement relative to one another. By mechanically adjusting the compensating members longitudinally relative to each other to thereby vary the distance between them, there is imposed on the optical grating a longitudinal strain of a magnitude that varies in a manner to balance out or compensate for wavelength variations resulting from changes in the temperature of the grating. This prior art temperature compensating waveguide device arrangement is, however, cumbersome and expensive to manufacture. Recently Corning Inc. has used a packaging substrate with negative thermal expansion coefficient in the fiber grating. The long-term reliability is being investigated. However, a more compact package with tunable mechanism and low temperature dependency is needed in the market.
It is a primary object of the present invention to provide a bimetal-based temperature stabilized multi-FBG package with tunable mechanism, in which very fine fiber grating can be obtained.
It is another object of the present invention to provide a bimetal-based temperature stabilized multi-FBG package with tunable mechanism, which is easy to use and the fiber therein will not be twisted
It is a further object of the present invention to provide a bimetal-based temperature stabilized multi-FBG package with tunable mechanism, in which the device is very compact.
It is a further object of the present invention to provide a bimetal-based temperature stabilized multi-FBG package with tunable mechanism, in which the grating length (Lf) can be adjusted directly by the moving pin to control the degree of over/under compensations, caused by fabrication imperfection.
According to the present invention, the device mainly includes a moving pin, a bimetal fixture, a rotation sleeve, a locking pin. The moving pin has a first predetermined outer screw pitch, for example 0.35 mm pitch, at one end and an elongated slot at the other end for receiving the locking pin. The bimetal fixture has a main frame and a plate secured to the main frame. The thermal expansion coefficient of the main frame is smaller than that of the plate. The main frame of the bimetal fixture has a tube member extending outwardly from the side wall thereof, and the tube member has a second predetermined outer screw pitch, for example 0.4 mm pitch, at the distal end thereof. A hole is formed on the side wall for receiving the locking pin. The rotation sleeve has a first predetermined inner thread corresponding to the first predetermined outer screw pitch of the moving pin and a second predetermined inner thread corresponding to the second predetermined outer screw pitch of the bimetal fixture.
The grating fiber is first placed inside the moving pin and then the fiber is metallized or soldered to the moving pin and the holding arm of the bimetal fixture. The slot of the moving pin is guided by the locking pin which enables the linear movement of the moving pin. The moving pin is disposed between the rotation sleeve and bimetal fixture with one end inserted into the bimetal fixture and the other end is screwed into the first predetermined inner thread, and then the second predetermined inner thread of the rotation sleeve is screwed onto the second predetermined outer screw pitch of the bimetal fixture. When the locking pin is in position, the moving pin cannot self-rotate, so rotating the sleeve in one cycle will make the moving pin has a movement of the second predetermined outer screw pitch minus the first predetermined outer screw pitch, that is, 0.4 mmxe2x88x920.35 mm=0.05 mm. Once the locking pin is rotated outwardly not to guide the slot, rotating the rotation sleeve 30 in 360 degrees will result in the second predetermined outer screw pitch (0.4 mm) movement of the moving pin, which called xe2x80x9cquick movementxe2x80x9d. The moving pin and the rotation sleeve are made by kovar with very low thermal expansion coefficient.
It should be appreciated that the above embodiment of the present invention provide the temperature-compensating devices with a tunable mechanism for optical fiber gratings, wherein the grating can be tuned with a very fine adjustment when the locking pin engages with the slot of the moving pin or the grating can be tuned with a relatively quick adjustment when the locking pin does not engage with the slot of the moving pin. In additions, a more compact package with a tunable mechanism and low temperature dependency is provided to overcome the disadvantages of the conventional devices.